The present invention relates to electroconductive polyurethane foam suitable for use as a material for rollers which electrostatically control the object to be contacted, such as toner transfer rollers, charging rollers, development rollers, and cleaning rollers used in the printer for electrophotography and electrostatic recording systems.
In the case of prior art electrophotograpbic systems in which toner contained in the toner cartridge is supplied to a photosensitive member where electrostatic latent images are formed to thereby transfer and fix images on paper, the system comprises mechanisms such as for (1) electrification, (2) exposure, (3) development, (4) transfer, (5) fixation, and (6) diselectrification. Each mechanism includes various types of rollers for precise control of the static electricity. Quality requirements for materials of such rollers are increasingly rigorous in recent years.
As rollers used in the development mechanism such as toner transfer rollers, electrified rollers, development rollers, transfer rollers, and rollers used in the cleaning mechanism function to electrically control the object to be contacted, electroconductivity of their material must be stable against changes in the environmental conditions. For example, fluctuation in the resistance should remain within the range of one digit order, e.g. between 1.times.10.sup.8 .OMEGA.-cm and 1.times.10.sup.9 .OMEGA.-cm, when the ambient conditions are between 5.degree. C. at 30% relative humidity and 30.degree. C. at 85% relative humidity.
Flexible polyurethane foam is usually used as the material for such rollers as these rollers should not damage the precision parts such as the photosensitive drum when contacted, or as the contact surface of the rollers must be increased for secure gripping. Moreover, because electrostatic control operation by these rollers is usually carried out in a very limited area, cell size of such polyurethane foam is required to be very fine.
As an electroconductive polyurethane foam useful as the material for such rollers, the following materials have been known in the art:
(1) polyurethane foam obtained by blending electroconductive carbon with a mixture of polyol, isocyanate, catalyst, water and foaming agent such as fluorocarbon, PA0 (2) polyurethane foam obtained as (1) above and added with ionic antistatic agent, and PA0 (3) polyurethane foam impregnated with carbon paint, etc.
With the conventional polyurethane obtained as (1) above, a larger amount of carbon addition is necessary to increase the conductivity (volume resistivity of 1.times.10.sup.9 .OMEGA.-cm or lower). This increases the viscosity of the system and results in uneven mixing of carbon with other components, thus making it difficult to obtain fine and uniform cells in the conductive polyurethane foam. Since very subtle difference in the carbon addition results in an extremely large difference in the conductivity, to control the conductivity at 1.times.10.sup.8 .OMEGA.-cm or higher in terms of volume resistivity, accurate control of the conductivity was difficult. Further, the conductivity (resistivity) of the resultant polyurethane foam is highly dependent on the environment.
Polyurethane obtained in (2) above is defective in that its conductivity (resistivity) is particularly dependent on the environment.
Polyurethane obtained in (3) above is defective in that in order to impregnate the carbon dispersion into the polyurethane foam, cells in the foam must be relatively large. If the cells are small, carbon particles will not penetrate into the foam (as the foam acts as a filter), and the conductivity will become uneven.